Traditional unboosted hydraulic-mechanical brake systems include a master cylinder that is connected via hydraulic lines with individual brake cylinders at each of the wheels to operate the vehicle wheel brake at that wheel. When the driver presses on the brake pedal, a master cylinder connected to the brake pedal pressurizes the hydraulic fluid, and the pressure is communicated through the hydraulic lines to operate the brake cylinders at the wheel brakes. The brake pressure resists the travel of the brake pedal, so that the driver directly feels the brake actuation.
It is also known to operate the vehicle brakes via an electro-hydraulic booster system in which the hydraulic pressure for operating the brake cylinders is provided by a hydraulic pressure accumulator that is charged by a hydraulic pump. In a typical electro-hydraulic system, pressing on the brake pedal will actuate a switch or sensor to energize an electrical controller that controls a boost valve to release pressure from the accumulator for communication through the hydraulic lines to operate the brake cylinders at the wheel brakes.
It has been recognized that an electro-hydraulic system is dependent upon a fault-free electrical and hardware system, and accordingly Federal Motor Vehicle Safety Standard 135 sets requirements to ensure safe braking performance during both normal conditions and emergency conditions. Thus, it is known in the design of electro-hydraulic brake booster systems to retain the use of a master cylinder as a back-up for the electro-hydraulic hydraulic pressure source. Typically then, upon the occurrence of a fault in the electro-hydraulics, or a switching off of the ignition key while the vehicle has not yet been shifted to park, the braking will immediately revert to the traditional unboosted hydraulic braking by reestablishing hydraulic fluid communication between the master cylinder and the wheel cylinders. Additionally, modern vehicles, particularly large trucks and luxury vehicles have many electrical systems drawing electrical current and it is accordingly necessary to shutdown these electrical systems, including the electro-hydraulic braking system quickly in response to turning off of the ignition key, so that the battery charge is not unduly drained.
In some instances, upon shutdown of the boosted brakes, and reversion to the unboosted hydraulic braking, the abrupt reestablishment can result in an abrupt feedback of the hydraulic pressure at the wheel brakes into the master cylinder and the brake pedal, disturbing the driver.
It would be desirable to provide improvements in the electro-hydraulic brake systems so that the transition from boosted electro-hydraulic control to unboosted master cylinder control will remain transparent to the driver.